Various types of distortion and noise are introduced into data signals that are transmitted over a communication path. The distortion and noise are due to interference with other signals within the same frequency range and also due to multipath dispersions. Multipath dispersions occur when signals propagate along different or reflected paths through a transmission medium to a receiving destination. In a wireless system, data signals are transmitted through space as modulated radio transmission waves or electromagnetic waves. Radio transmissions or data signals that travel through the air can reflect off trees, vehicles, houses, larger buildings, and terrain features such as mountains and hills. Generally, a signal or beam travels along a main or direct line-of-sight transmission path, while reflected signals travel along various reflected paths. Each reflected path has an associated delay and the overall effects of all such signals are a combination of the main signal and a plurality of reflected or delayed signals. Therefore, the signal received is usually not the same as the original signal transmitted, and when the signal is demodulated and decoded, errors in the original transmitted data often result.
The effect of the multipath scattering is to alter or distort the received signal spectrum when compared to the spectrum as transmitted. In general, the effects are different at various frequencies across the signaling band. At some frequencies, the multipath signals add constructively to result in an increased signal amplitude, while at other frequencies the multipath signals add destructively (out of phase) to cancel or partially cancel the signal, resulting in reduced signal amplitude.
A wireless communication system is designed to compensate for the deleterious effects of multipath dispersion. Many wireless systems and some wired systems employ a channel estimation procedure to determine the effects the transmission environment has on the transmitted data signals. The channel estimation procedure can utilize training signals of known magnitude and phase to compensate for signal changes due to the transmission environment. The training signals can be transmitted prior to transmission of the data signals or interspersed in the data signals. The training signals can be analyzed to determine the effects of the environment on the transmitted signal and this information utilized to adjust the data signals appropriately.
One mechanism for analyzing training signals is to perform an Inverse Fast Fourier Transform (IFFT) on the training signals to determine the impulse response of the communication channel. If there is little or no distortion in the channel, then one sample of the channel impulse response will have a significant value, while the remaining samples will have little or no value. If there is distortion in the channel, then each sample of the impulse response will have some significant value. The impulse response is then used to determine the magnitude change and the phase shift that the environment has caused to the data signals. An adjustment can then be made on the data signals, so that a reduction in transmission errors can be facilitated.
However, the employment of FFTs and IFFTs on signals is computationally expensive. Therefore, any reduction in system computations is desirable.